Bud Light's shrinking sales have prompted some of its distributors to cut back on marketing the beer, threatening to exacerbate troubles at the Anheuser-Busch InBev SA brand.

Distributors for years complained that the company wasn't taking the right steps to stem declines at Bud Light. Though it continues to be America's best-selling beer, Bud Light has been losing ground to Mexican lagers, craft brews, wine and harder liquor. Now, with margins shrinking, some are moving to protect profits.

"Wholesalers are wrestling with a decision right now," said David Stokes, who owns Grey Eagle Distributors, and whose father once served as chief executive of Anheuser-Busch. "Do you continue to invest behind Bud Light and make a little bit less money right now to build for the long term, or do you cut costs to try to maintain the health of the bottom line?"

While sales of other "light lagers" like Molson Coors Brewing Co.'s Coors Light and Miller Lite also declined, the fall at Bud Light has been steepest. The brand suffered its biggest volume drop ever last year--down an estimated two million barrels, or 5.7%. AB InBev has invested heavily in small-brewery beers it brands as craft, as well as foreign labels like Stella Artois and its higher-end light lager Michelob Ultra. But for many of AB InBev's 600 or so distributors, Bud Light is still their top seller.

Distributors, or wholesalers, are the middlemen who sell beer to retailers like grocery stores and restaurants. For a big brewer like AB InBev, the relationship with these mostly independent businesses is crucial--similar to an auto maker's reliance on its network of car dealerships. AB InBev funds big-budget national advertising campaigns, but it depends on its distributors to run regional promotions and work closely with retailers to push more beer.

Until now, these wholesalers have been able to trim operational costs and increase prices to cushion the bottom line. But falling sales of Bud Light are hurting their profits, leading some to cut marketing budgets.

Falling Bud Light and Budweiser volumes sent pretax earnings per case down 2% last year at the Florida operations of Gold Coast Eagle Distributing, according to John Saputo, the distributor's owner and president. He said earnings fell 5% per case at his Ohio business. Both declines came despite rising sales of Michelob Ultra and Stella Artois.

At Straub Distributing Co., based in Orange County, Calif., Chief Executive Mark Danner said case sales of Bud Light--which account for about 45% of all case sales--were down by "high single digits" last year. He cut back on Bud Light marketing at concerts and on billboards.

A large distributor in the Midwest is cutting its regional marketing spending on Bud Light, which accounts for a third of the cases it sells, according to a person familiar with the matter. That is after weathering five years of annual volume declines, during which the distributor kept spending steady, the person said.

AB InBev is taking steps to appease distributors. Last year, it brought back a Bud Light veteran to lead the brand, emphasizing outreach to distributors. The company is rolling out new fruit-flavored versions brewed with lime and orange peels.

It also is boosting national advertising. AB InBev said it increased its Bud Light media spend by 5% in the fourth quarter.

At this weekend's Super Bowl, it plans to debut a new spot in its latest campaign--about a medieval king with a penchant for Bud Light and his made-up toast, "Dilly Dilly."

The phrase became a national meme after Pittsburgh Steelers quarterback Ben Roethlisberger shouted it before a snap in November. Dilly-Dillies now regularly flood Twitter after NFL touchdowns.

Whether it can move more beer is an open question. After the TV spots started running in August, monthly volume declines for Bud Light slowed, but then sharpened again toward the end of the year, according to Nielsen.

"We cannot forget Bud Light is the leading brand in the industry," said Marcel Marcondes, AB InBev's U.S. marketing vice president, in a recent interview. "It's not a detail, it's a top priority." Last year, he was involved in tapping Andy Goeler, who ran the brand in its heyday in the 1990s, to turn things around. Mr. Goeler said his priority is keeping wholesalers behind Bud Light.

"I want to get Bud Light stabilized so they can stay focused on us and stay with us as partners," Mr. Goeler said. While the Dilly Dilly ads haven't increased volumes, they have boosted sentiment toward Bud Light on social media, he said. He expects that to translate into higher sales during the warmer months. "The focal point of what we're trying to do is to get Bud Light into the conversation, make it culturally relevant," he said.

Some distributors said Mr. Goeler already has been more effective connecting Bud Light's marketing campaigns on television and social media to the regional billboards wholesalers put up and the displays they place in grocery-store aisles. They also praised another recent marketing campaign that touted the simplicity of Bud Light while poking fun at more complex beers.

Mr. Goeler said the Dilly Dilly spots are helping "bring back the fun" at Bud Light. In November, he paced around a Missouri hotel conference room, performing the scripts of four new spots in development for the campaign, according to attendees. A group of wholesalers joined in when Mr. Goeler pumped his index finger in the air for the campaign's signature line.

Wholesalers were initially skeptical of the Dilly Dilly ad that was created as part of six rotating spots by Bud Light's ad agency over the summer. "Childish and ridiculous," Devyn Dugger, president of Ohio Eagle Distributing and a member of AB InBev's wholesale advisory panel, remembers thinking when he saw them. But "look where it's going," he said. "I've been fully converted."